It's just a random thought I had earlier today, how come all images on the computer are rectangular/square?
It seems like if you were to draw a circle and wasn't expecting to use any of the outside of the circle, you would be wasting space. Granted you could leave it transparent, but what's the point of it being there in the first place?
I think it is a way to show how much space you have to do something in. Like you have a square you could draw a circle inside and only work inside of the circle, but you know you have that extra space if you want to make the circle bigger or you want to go outside of it. If not then you can just make the outside transparent and no one will know.
Easier to represent as bits in a grid. Keep in mind an image is represented as pixels on a coordinate plane. (well unless it is a vector image) Sure you could have a circle file but it would probably take up the same space as a square file with a circle in it so there is no real advantage.
In response to Asielen
You could represent a circle in a different type of grid. ;p
Center x Radius(Distance from center to edge)

But, as for the more complicated objects, I would have to agree with you, that would suck! Still, it seems like it would be neat, maybe someone will make images like this someday.